As electrooptical apparatuses using a liquid crystal, electrooptical apparatuses using nematic liquid crystals such as a DSM type display, TN type display, G-H type display, or STN type display have been developed and practically used. However, all of the electrooptical apparatuses using such nematic liquid crystals suffer the drawback response time is as slow as several milli sec to several tens milli sec, which results in a limited range of applications. The slow response of the electrooptical apparatuses or elements using nematic liquid crystals is due to the fact that the torque which changes the direction of molecules is inherently based on the anisotropy of dielectric constant and thus, the force is not so strong. With such a technical background, the development of a ferroelectric liquid crystal had been attempted which has a spontaneous polarization (Ps), has a strong torque based on Ps.times.E (E is an applied voltage), and has an extremely short optical response time of few .mu.sec to several tens .mu.sec to make the preparation of a ultrahigh speed device possible.
Mayer et al. synthesized DOBAMBC (p-decyloxybenzilidene-p-ammino-2-methylbutyl cinnamate) in 1975 for the first time in the world and which was confirmed to be a ferroelectric liquid crystal (Le Journal de Physique, Vol. 36, 1975, L-69).
Further, since Clark and Lagerwall reported in 1980 on such characteristics on display devices as high velocity response of submicroseconds and memory characteristics of DOBAMBC, ferroelectric liquid crystals have drawn considerable public attention (N. A. Clark et al., Appl. Phys. Lett. 36, 899 (1980)).
However, many technical problems in the above mentioned system have presented obstacles to its practical application. In particular, no material was reported as exhibiting ferroelectric liquid crystallinity at an ambient temperature. Moreover, an effective and practical method was not established to control the molecular alignment of the liquid crystal molecules. Control of the molecular alignment is essential in order to have an effective and practical liquid crystal display device.
After the publication of the report, various attempts have been made from both aspects of liquid crystal materials and device, display devices utilizing the switching between twisted bistable states were prepared for trial, and high speed electrooptical apparatuses using the device are proposed in U.S. Pat. No. 4,367,924 and others. However, high contrast and proper potential of threshold value have not been obtained.
From such a point of view, other switching systems were explored to propose a transitional diffusion system. Subsequently, a three states switching system of liquid crystal having tristable states was reported in 1988 (A. D. L. Chandani, T. Hagiwara, Yo Suzuki et al., Japan, J. of Appl. Phys., 27, (5), L729-L732 (1988)).
The optically tristable states herein referred to mean that, when voltage in the form of a triangular wave as in FIG. 1 A is applied to liquid crystal electrooptical devices where antiferroelectric liquid crystals are laid between the first electrode substrate plate and the second electrode substrate plate which is apart at a given space from the first one, the antiferroelectric liquid crystal shows the first stable molecular orientation and resulting the first optically stable state shown in FIG. 3 (a), and FIG. 1 (D) at point 2, respectively, when electric voltage is zero. The antiferroelectric liquid crystal shows the second stable molecular orientation and resulting the second optically stable state shown in FIG. 3 (b), and FIG. 1 (D) at point 1, respectively, in one of the direction of electric field and shows the third stable molecular orientation and resulting the third optically stable state shown in FIG. 3 (c), and FIG. 1 (D) at point 3, respectively, in the other direction of electric field.
Liquid crystal electrooptical apparatuses utilizing the tristable states, that is three states, are proposed in U.S. Pat. No. 5,046,823 filed by the present applicant.
The characteristics of an antiferroelectric liquid crystal showing the tristable states are described in more detail below.
In the ferroelectric liquid crystal element having a stabilized surface which was proposed by Clark-Lagawall, ferroelectric liquid crystal molecules show two stable states in which the molecules are uniformly oriented or aligned in one direction in the phase S*C. The molecules are stabilized in either state depending on the direction of applied electric field as shown in FIG. 2 at point (a) and at point (b), and the state is kept even when the electric field was shut off.
Actually, however, the alignment of the ferroelectric liquid crystal molecules shows twisted two states in which directors of the liquid crystal molecules are twisted or shows a chevron structure in which layers are bent in a doglegged shape. In the chevron layer structure, switching angle becomes small, forming a cause for a low contrast, and which constitute a serious obstacle for its practical use.
On the other hand, in the liquid crystal electrooptical devices, an "anti" ferroelectric liquid crystal molecules are aligned in antiparallel, tilting in opposite direction at every adjoining layer, in the phase S*.sub.(3) showing the tristable states, and thus, the dipoles of the liquid crystal molecules are negating each other. Accordingly, the spontaneous polarization is nullified as a whole. The transmittance of the liquid crystal phase showing such molecular alignment corresponds to point 2 in FIG. 1 D.
Further, when a voltage sufficiently higher than a threshold value of (+) or (-) was applied, liquid crystal molecules shown in FIG. 3 (b) or (c) are tilted in the same direction and aligned in parallel. In this state, the spontaneous polarization is produced since the dipoles are also shifted to the same direction to form a ferroelectric phase, and the transmittance of the liquid crystal phase in that state corresponds to points 1 and 3 in FIG. 1 D.
That is, in the phase S*.sub.(3) of the "anti" ferroelectric phase, the "anti" ferroelectric phase at the time of no-electric field and two ferroelectric phases due to the polarity of applied electric field are stabilized, and switching is carried out among tristable states of an "anti" ferroelectric phase and two ferroelectric phases, with a direct current-like threshold value. Based on the change in the alignment of liquid crystal molecules accompanied with the switching, light transmittance is changed while drawing such a double hysteresis as shown in FIG. 4.
One of the characteristics of the present invention is that a memory effect can be realized by applying a bias voltage to the double hysteresis as shown in FIG. 4 (A) and then, further applying a pulse voltage.
Moreover, the ferroelectric phase is stretched in terms of its layer by the application of an electric field to form a book-shelf structure. On the other hand, in the "anti" ferroelectric phase of the third stable state, an analogous book-shelf structure is formed. Since the layer structure switching due to the application of an electric field gives a dynamic shear to liquid crystal layers, an alignment defect is improved during driving, and thus, a good molecular alignment can be realized.
In the "anti" ferroelectric liquid crystal, since image display is performed by alternatively using both hysteresises of plus side and minus side, afterimage phenomenon due to the accumulation of inner electric field based on the spontaneous polarization can be prevented.
As explained above, the "anti" ferroelectric liquid crystal can be said to be a very useful liquid crystal compound having advantages as follows:
1) Ultrahigh speed response is possible,
2) High contrast and wide viewing angle can be expected, and
3) Excellent alignment characteristics and memory effect can be realized.
Reports are made on the liquid crystal phase of the "anti" ferroelectric liquid crystal showing the tristable states in the following articles:
1) A. D. L. Chandani et al., Japan J. Appl. Phys., 28, L-1265 (1989), and
2) H. Orihara et al., Japan J. Appl. Phys., 29, L-333 (1990).
The liquid phase is called "Phase S*.sub.CA " (Antiferroelectric Smectic C phase) in association with the "anti" ferroelectric property. The phase is named "phase S*.sub.(3) " in the present specification since the liquid crystal phase performs the switching among tristable states.
The liquid crystal compounds which have the "anti" ferroelectric phase S*.sub.(3) showing the tristable states in a phase series are disclosed in Japanese Unexamined Patent Publication No. 1-316367, U.S. Pat. Nos. 5,171,471 and 4,973,738, and European Patent No. 330,491 A filed by the present inventors, and in Japanese Unexamined Patent Publication No. 1-213390 filed by Ichihashi et al. Liquid crystal electrooptical devices utilizing the tristable states are proposed in Japanese Unexamined Patent Publication No. 2-40625 and U.S. Pat. No. 5,046,823.
The liquid crystal compounds having a thioester linkage are reported in Japanese Unexamined Patent Publication Nos. 63-222147 and 3-44367, and U.S. Pat. No. 5,068,054.
However, in Japanese Unexamined Patent Publication No. 63-222147, optically active alcohols such as 2-methyl pentanol are used while the liquid crystal phase shows a ferroelectric peoperty.
Besides, the liquid crystal compound in Japanese Unexamined Patent Publication No. 3-44367 has a thioester linkage in a side chain of its molecular structure and does not show a ferroelectric property, being different from the compound of the present invention.
Further, in U.S. Pat. No. 5,068,054, optically active alcohols such as 2-alkanol are used while the liquid crystal compound has a thioester skelton and shows a ferroelectric property.
As will be understood from the above, publications have disclosed neither a chemical structure in which a fluoroalkyl group is bonded to an asymmetric carbon atom nor such a ferroelectric liquid crystal phase S*.sub.(3) as the compound of the present invention.